Power conversion refers to the conversion of one form of electrical power to another desired form and voltage, for example converting 115 or 230 volt alternating current (AC) supplied by a utility company to a regulated lower voltage direct current (DC) for electronic devices, referred to as AC-to-DC power conversion, or converting. Power converters are included as part of the chargers and adapters used by electronic devices such as mobile phones, tablets, laptops, and other mobile electronic devices.
As mobile electronics devices continue to grow in popularity, there are increasing demands for miniaturization (high power density) and portability. In order to achieve such miniaturization and portability, higher switching frequency and higher efficiency are demanded. The size of a power converter is generally related to the device switching frequency and efficiency. A higher switching frequency can decrease the size of energy storage components such as electromagnetic components and electrostatic components. Higher efficiency can decrease the size of a heat sink needed to cool the device. As such, high frequency and high efficiency are future trends in the electronics technology.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, an SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage. The switch mode power supply uses the high frequency switch, the transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter.
In many applications where SMPS are used the requirement for accurate current measurement of the current from the SMPS to the load have been constantly increasing. Traditionally the current has been measured over a resistive shunt, which has provided accurate measurement when the shunt has been a discrete element with a defined resistance.
As the power consumption in the applications has increased, sensing over components used in the conversion process has been used, called DCR (DC Resistance) sensing. This constitutes sensing the current over the resistive part of an inductor (sensing element) or the negative current return path in the PCB. This type of sensing typically offers a tolerance, or variation, of 7 to 10 percent, which is higher than normally required in a modern application. Another method is to integrate the sensing element into an integrated power circuit, which is a part of the power conversion process. This type of circuit is only available for certain applications and not applicable to all topologies.
The concept of resistive trimming using DCR sensing has been used for many years. Trimming reduces the variation of the resistive part of the sensing element, such as the inductor in the SMPS. The variation can be compensated for, and thereby reduced if not eliminated, through the use of a digital controller. Such compensation can be performed by first reporting the value of a current being drawn from the device (SMPS) using a non-calibrated coefficient K in the digital controller. The value is reported to a measurement system, which compares the current read by the SMPS with a reference current. The coefficient K is then updated by multiplying K by the ratio of the reference current and the current reported by the SMPS.
In other applications, a digital controller is not included. One method to calibrate the current in a device not having a digital controller is to use a resistive divider over the sensing element, where one of the resistors in the resistive divider is trimmed to a desired value using trimming equipment. This method relies on a current being drawn through the device during the entire trimming process. FIG. 1 illustrates an exemplary simple SMPS circuit schematic to which DCR sensing is applied. The input voltage, V11, is pulse width modulated (PWM) by transistors T11 and T12. The resulting PWM waveform at switching node SW is then averaged to a DC voltage by the filter formed by inductor L11 and capacitor C11. Current provided to a load is represented as I11. The SMPS circuit shown in FIG. 1 does not include a digital controller for implementing DCR sensing. Instead, a resistive divider including resistors R11 and R12 is coupled across the inductor L11 for implementing DCR sensing. The inductor L11 is considered the sensing element and includes both an inductance part L and a resistance part RL. The SMPS is operated so as to generate a PWM signal at the switching node SW, and current is drawn through the branching current pathways to the load, specifically branching at the inductor L11 and the resistor R11, and branching at the capacitor C12 and the resistor R12. The characteristics of the resistive divider components R11, R12, C12 are chosen such that a cutoff frequency of the RC-filter R11, R12, C12 is matched to a cutoff frequency of the inductor L11. With such a configuration, a voltage over the capacitor C12, and therefore also over the resistor R12, is proportional to the voltage over the resistance part RL, which enables indirect measurement of the current through the resistive part RL. The resistive divider formed by resistors R11, R12 allows trimming of the value of the resistive part RL of the inductor L11, and can be achieved by trimming of either of the resistors R11 or R12. In the example shown in FIG. 1, the resistor R12 is a trim resistor. To trim the trim resistor R12, the device is turned ON thereby drawing current through the device, and the resistor R12 is trimmed to a desired value according to a monitored voltage measurement taken between the nodes at test point TP and Vout. A problem with this technique is that the trimming process is relatively slow, e.g. in the range of seconds, and a relatively large current, e.g. 10-50 amps, is required to pass through the sensing element (inductor L11) during the entire trimming process. The trimming time is long enough that the resistive part RL of the sensing element increases in temperature as current is run through it, referred to as self-heating, and the increased temperature significantly reduces the obtainable accuracy of the trimming procedure. In particular, since the resistivity of the sensing element (inductor L11) has a large temperature coefficient, the resistance value of the resistance part RL will change during the trimming process. This can be remedied by using a temperature compensation unit coupled to the sensing element. However, there is a mismatch between the thermal time constant of the sensing element and the thermal time constant of the temperature compensation unit, resulting in the sensing element RL being compensated not having the same temperature as the thermal sensor. As such, compensation applied according to the temperature compensation unit is not completely accurate. The mismatched time constants, together with the requirement on the trimming system to constantly update its target value, since the self heating of the sense element will result in the target voltage having to be updated depending on the sense element temperature, makes this method highly unreliable. Also, since the actual time to perform the trimming may vary from device to device, there is a corresponding variance in heat generated over the sensing element (inductor L11), resulting in varied trimmed resistance values from device to device.